This invention relates to plastic injection molds for molding containers having multiple peripheral flanges, and in particular, to means for stripping or ejecting the molded containers upon opening of the mold.
Containers, such as buckets or pails often have an enlarged peripheral rim or lip and a second annular flange spaced below the rim for strengthening purposes, or for attaching a bail or handle to the container. A common way of molding such a container is to provide collets in the mold that open radially outwardly to release the container rim and flange and thus allow the container to be ejected or stripped from the mold core.
A difficulty with collet-type molds is that they are complex and have a lot of sliding components which are prone to wear problems.
An alternative approach which eliminates some of the difficulties of the collet-type molds is shown in U.S. Pat. No. 4,648,834 issued to John W. Von Holdt. In this Von Holdt patent, two axially movable concentric stripper rings are used. A first stripper ring molds the container rim and the second stripper ring engages the container flange. When the container is molded and the mold opens, the second ring is moved upwardly against the container flange to lift the container off the mold core. The first ring floats along with the container rim until the container rim shrinks inwardly upon cooling to release itself therefrom.
A difficulty with the above-mentioned Von Holdt structure, however, is that it depends upon the type of plastic and the molding parameters such as temperature, molding time, wall thicknesses, speed of withdrawal of the container from the mold, etc. in order to work properly. It is hard to control precisely all of these parameters.
The present invention is an improvement over the Von Holdt structure in that two driven concentric ejector rings are used to precisely eject the molded container without having to depend on shrinkage due to cooling for proper operation.
According to the invention, there is provided an injection mold comprising a core portion and a cavity portion defining therebetween a mold cavity in the shape of a container to be molded, the container having an enlarged annular rim and an annular flange spaced therefrom. An inner, annular ejector ring is located in the mold core portion. The inner ejector ring defines a portion of the enlarged annular rim. An outer, annular ejector ring is coaxially located adjacent to the inner ejector ring. The outer ejector ring defines a portion of the annular flange. Axially movable first push rod means is connected to the inner ejector ring for moving the inner ejector ring outwardly to partially eject a molded container from the mold core portion upon opening of the mold. Also, axially movable second push rod means is connected to the outer ejector ring for moving the outer ejector ring outwardly relative to the inner ejector ring to complete the ejection of the molded container from the mold core.